Apertureless near-field scanning optical microscopy (ANSOM) provides spatial resolution beyond the diffraction limit of light. However, there is a key issue with respect to ANSOM to resolve for some spectroscopic applications, namely: the directly detectable near-field scattering signal has dispersive profiles rather than absorptive profiles. The absorptive profiles are generally a collection of peaks on spectrum that are obtained from infrared (IR) absorption spectroscopy. Far field spectral banks with absorption profiles characteristic of chemical compounds have been created for chemical identifications. As a result, ANSOM is not yet a convenient tool for infrared nanospectroscopy due to its dispersive profiles and is unable to tap into the existing spectral banks that characterize macroscopic samples of materials to achieve nanoscale chemical identification.
There are currently several ways to obtain the desired absorptive profile in ANSOM implementations. One recent technique is called pseudoheterodyne detection, and it relies on lock-in detection on the combination reference frequency of ANSOM tip oscillation and reference phase modulation (see Ocelic, N., A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free near-field spectroscopy” Applied Physics Letters, 2006. 89(10)). While it obtains the phase of near-field scattered light, there are two disadvantages associated with the pseudo-heterodyne technique. The first one is the reduced signal level due to detection of weak sidebands instead of the main band in lock-in detection. The second potential drawback is that it measures the phase of near-field scattered signal instead of the imaginary part of the near-field. The equivalency of phase and imaginary part of near-field scattered signal is a good approximation under the condition that the real part of near-field scattering is strong, but the approximation is not always correct.
The second technique to obtain absorptive profile is to use a coherent broadband light source to do asymmetric Fourier transform infrared spectroscopy. This has been described in two articles, see Huth, F., et al., “Nano-FTIR Absorption Spectroscopy of Molecular Fingerprints at 20 nm Spatial Resolution”, Nano Letters, 2012. 12(8): p. 3973-3978; and Xu, X. G., et al., “Pushing the Sample-Size Limit of Infrared Vibrational Nanospectroscopy: From Monolayer toward Single Molecule Sensitivity” The Journal of Physical Chemistry Letters, 2012. 3(13): p. 1836-1841. While it offers multiplex technique capability, this type of technique requires a coherent broadband light source that is expensive (˜$300 k in 2012) and comes with low laser energy that intrinsically leads to low signal quality, which limits its practical applications.